Development of a shear band cleavage as a result of strain partitioning

Microstructural analyses of shear band cleavages in a centimeter-scale shear zone within a metasomatic biotite band in the Teshima granite, Ryoke metamorphic belt, southwest Japan, show that strain partitioning occurred between quartz and biotite-feldspar domains within the shear zone. Pre-tectonic hydrothermal alteration within the granite caused biotite replacement of both plagioclase and K-feldspar, resulting in the development of biotite-feldspar domains where K-feldspar mantles dominantly biotite-plagioclase aggregates. Subsequently, the altered granite was plastically deformed in simple shear, so that intra-layer shear band cleavages were passively developed within the biotite-feldspar domains, whereas intense dynamic recrystallization occurred in the quartz domains. The rotation and orientation of the intra-layer shear band cleavages can be explained by a finite strain ellipse model, which shows that strain in the biotite-feldspar domain requires only 10–20% of the bulk simple shear strain for the development of such cleavages, so that most of strain could be accommodated by deformation in the quartz domain. Consequently, the model suggests that the development of the shear zone resulted in strain partitioning between the quartz and the biotite-feldspar domains due to compositional variations via hydrothermal alteration within the granite.